Judging from the post so far and a private response I got, I realise that I was not very clear in my first post.
I am not sure whether the input level sliders work on the analog or the digital part of soundcards, i.e. before or after the ADC, but having as a working hypothesis that the mixer works on the analog circuit, what I do is: a) match my preamplifier console output level and the soundcard input (mixer slider) so they give a little above 0db b) lower gradually *with the mouse* the mixer slider until no clipping occurs.
In these terms what I desire is a software solution to lowering the slider i.e. a program to do what my hand does when lowering the slider, in the very same sense that MAD does it - when it sees clipping it reduces the "amplification".
In the hope that I am more clear now, I wait for any further ideas :)